1. Field of the Disclosure
The present invention relates to a thermal ink jet printer and, more particularly, to a system and method for sensing the operating temperature of a print head.
2. Related Prior Art
Ink jet printers eject ink onto a print medium such as paper in controlled patterns of closely spaced dots. To form color images, multiple ink jet print heads are used, with each head being supplied with ink of a different color. Thermal ink jet printing systems use thermal energy selectively produced by resistors located in ink filled channels. Firing signals are applied to the resistors through associated drive circuitry to vaporize momentarily the ink and form bubbles on demand. Each temporary bubble expels an ink droplet and propels it toward a recording medium. The printing system may be incorporated in either a carriage type printer or a page width type printer. A carriage type printer, such as the type disclosed, for example, in U.S. Pat. No. 4,571,599 and U.S. Pat. No. Reissue 32,572, generally includes a relatively small print head containing ink channels and nozzles. The contents of these patents are hereby incorporated by reference. The print head is usually attached to a sealed ink supply and the combined print head and ink supply form a cartridge assembly which is reciprocated to print one swath of information at a time on paper that is held stationary. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, so that the next printed swath will be contiguously printed. The procedure is repeated until the entire page is printed. The page width printer has a stationary print head having a length equal to or greater than the width of the paper. The paper is continually moved past the page width print head in a direction normal to the print head length at a constant speed during the printing process. An example of a page width printer is found in U.S. Pat. No. 5,221,397, whose contents are hereby incorporated by reference.
A known problem with thermal ink jet printers is the degradation in the output print quality due to increased volume of ink ejected at the print head nozzles resulting from fluctuations of print head temperatures. These temperatures produce variations in the size of the ejected drops which result in the degraded print quality. The size of ejected drops varies with print head temperature because two properties that control the size of the drops vary with print head temperature: the viscosity of the ink and the amount of ink vaporized by a firing resistor when driven with a printing pulse. Print head temperature fluctuations commonly occur during printer startup, during changes in ambient temperature, and when the printer output varies.
When printing text in black and white, the darkness of the print varies with print head temperature because the darkness depends on the size of the ejected drops. When printing gray-scale images, the contrast of the image also varies with print head temperature because the contrast depends on the size of the ejected drops. When printing color images, the printed color varies with print head temperature because the printed color depends on the size of all the primary color drops that create the printed color. If the print head temperature varies from one primary color nozzle to another, the size of drops ejected from one primary color nozzle will differ from the size of drops ejected from another primary color nozzle. The resulting printed color will differ from the intended color. When all the nozzles of the print head have the same temperature but the print head temperature increases or decreases as the page is printed, the colors at the top of the page will differ from the colors at the bottom of the page. To print text, graphics, or images of the highest quality, the print head temperature must remain constant.
Various print head temperature controlling systems and methods are known in the prior art for sensing print head temperature and using sensed temperature signals to compensate for temperature fluctuations or increases.
U.S. Pat. No. 4,910,528 discloses an analog temperature sensing system wherein a thin film temperature resistor is codeposited on a substrate with the resistors that are heated to expel ink droplets from print head nozzles. The voltage drop outputs across the temperature resistor are sent to a temperature prediction circuit that controls the print strategy to maintain the print head temperature within a predetermined operating range.
U.S. Pat. No. 5,075,690 discloses an analog temperature sensor for an ink jet print head, which achieves a more accurate response by forming the thermistor on the print head substrate, and of the same polysilicon material as the resistors that are heated to expel droplets from the print head nozzles relative to ink temperature.
U.S. Pat. No. 5,220,345 discloses a print head temperature control system that places a plurality of temperature detectors at different positions and monitors the temperature differences to control ink supplied to the associated ink channels.
U.S. Pat. No. 5,315,316 discloses a print head temperature control circuit that includes a temperature sensor formed on the print head substrate. Analog signals from the sensor are delayed and analyzed by a data processor. A temperature summing operation is performed during a print operation and the sum is compared to a previously stored value to determine whether ink flow through the print head is sufficient for continued printing.
The device described in U.S. Pat. No. 5,172,142 senses changes in a temperature sensor to change the driving frequency of the print head. Analog signals from the sensor are converted into digital signals that are sent to a sequence controller for controlling operation of a pulse motor driver.
U.S. Pat. No. 5,168,284 discloses a closed loop system, which produces non-printing, pulses in response to a difference between a reference temperature signal and print head temperature signals produced by a temperature sensor located on the print head.
U.S. Pat. No. 5,223,853 to Wysocki et al. discloses a method of controlling the spot sizes printed by a thermal ink jet printer. The temperature of the ink in the print head is sensed. A combination of power level and time duration of the electrical input signal to the heating elements is selected based on the sensed temperature. A predetermined function relates the energy of the input signal to the corresponding resulting size of the spot on the copy sheet.
According to the prior art the preferred solution is to imbed an electronic temperature sensor within the print head. Problems arise because of sensor design, the difficulties of calibration, and changes due to mounting stress, encapsulation shifts, vibration, noise and other influences. To provide absolute calibration, further solid state sensors are being considered. It is a purpose of the system of this application to provide a sensing device that can be reliably calibrated prior to installation and that is more cost effective than solid state devices. It is another object of the system of this invention to use a liquid crystal type temperature sensor to provide this function.
Thermal ink jet printers are sensitive to temperature fluctuations at the print head. As a result the control circuitry is designed to respond to temperature sensed at the print head. The temperature sensors that are used are generally imbedded in the print head substrate to sense the approximate temperature of the ink. The output of such sensors is processed to provide digital signals, relative to ink temperature, to the microprocessor controller for the system.
In the system of this application, a liquid crystal temperature sensor is applied to the exterior of the print head cartridge. Liquid crystal temperature sensors give an optical indication of ambient temperature that is readable by an optical scanner. Since in many applications the print head cartridge is already optically scanned, the temperature sensor of this application may be readily integrated into existing designs. The liquid crystal temperature sensor is applied to the print head cartridge adjacent to the print head brand code to enable the optical reader to scan the temperature indication in addition to the brand code. Through appropriate circuitry the temperature signal from the sensor may be isolated and converted to digital form for use by the print controller.
In the printing operation either the print head is moved relative to the paper, or the paper is moved under the print head. In either instance there exists relative movement between the print head and adjacent structure. The optical reader is mounted to read the temperature indication during such relative motion.